1. Field of the Invention
The present invention relates to nonwoven fabrics and more particularly to nonwoven fabrics which are excellent in heat resistance, hot water resistance and steam resistance (hereinafter referred to as "heat-resistant characteristics") and further excellent in organic solvent resistance, acid resistance and alkali resistance (hereinafter referred to as "chemical-resistant characteristics"), and which are suitable particularly for medical fabrics, industrial filters, battery separators, and so forth.
2. Description of Related Arts
Nonwoven fabrics now used as industrial filters, battery separators and so forth, are made of polyolefins, polyesters or polyamides. In fact, however, nonwoven fabrics excellent in both heat-resistant characteristics and chemical-resistant characteristics have not been prepared; for example, nonwoven fabrics of polyolefins are poor in heat resistance, and nonwoven fabrics of polyesters or polyamides are poor in hot water resistance and steam resistance.
The present inventors' group has proposed styrene-based polymers with mainly syndiotactic configuration which are crystalline, have a high melting point and are excellent in chemical-resistant characteristics (Japanese Patent Application Laid-Open No. 104818/1987), and further stretched moldings (Japanese Patent Application Laid-Open No. 77905/1988) and fibrous moldings (Japanese Patent Application No. 4922/1988) both using the above syndiotactic styrene-based polymers.
However it has been found that nonwoven fabrics produced using the above styrene-based polymers as such are poor in heat-resistant characteristics and chemical-resistant characteristics; that is to say, excellent heat-resistant characteristics and chemical-resistant characteristics characteristic which the syndiotactic styrene-based polymers originally have are not exhibited when formed into nonwoven fabrics. Fibers obtained by extruding the above styrene-based polymers and then cooling are amorphous. Nonwoven fabrics made of the amorphous fibers sometimes shrink to enlarge the diameter thereof, or crystallize to become brittle, if used at temperatures higher than the glass transition temperature. Moreover the nonwoven fabrics are poor in chemical-resistant characteristics.
In order to overcome the above problems, an attempt to stretch the syndiotactic styrene-based polymer fibers by heating has been made. It has been found, however, that this stretching method readily causes fiber cutting, thereby failing to overcome the problems, and furthermore that the method is difficult to carry out on a practical scale in view of its operation process.